This invention relates to a ballast for high-intensity gas is charge lamps. More specifically, it relates to the reduction of vertical color segregation in a high intensity gas discharge lamp by the use of sequential excitation of the input power or the use of frequency sweep in combination with amplitude modulation.
Discharge lamps have been operated in pulsed mode, as illustrated in U.S. Pat. No. 4,904,903.This patent teaches methods of operating fluorescent mercury vapor, sodium and metal halide (MH) lamps, so that the input is electronically, periodically gated for a portion of the wave period. This pulsed operation is effective in eliminating undesirable electromagnetic and radio interference emissions.
Color control of high-intensity discharge (HID) lamps by pulsing techniques is also well known as shown by U.S. Pat. Nos. 4,137,484, 4,839,565, and 4,963,796. Japanese Patent No. 432153 teaches the use of exterior temperature regulation to control the color of the discharge lamp. Other color-controlling methods include interior temperature regulating techniques and varying the salts within the discharge tube.
One of the major problems in the operation of discharge lamps is the deformation of the arc within the discharge tube by convective gas flow. Techniques for stabilizing and centering this arc have been developed. U.S. Pat. No. 5,134,345 illustrates a method of avoiding acoustic frequencies that cause destabilizing phenomena. The method of this patent teaches the detection of arc instabilities, and changing the drive frequencies that cause them.
In U.S. Pat. No. 5,306,987, an arc stabilization technique is illustrated in which the frequency of the drive signal is modulated. A similar method of controlling the arc in discharge lamps is illustrated in U.S. Pat. No. 5,198,727. With this method, the arc is centered by the "acoustic perturbations" induced by the frequency of the drive signals. The acoustic perturbations compel the gas or vapor movement patterns to counter the gravity-induced convection.
U.S. Pat. No. 5,684,367 discloses a method of controlling arc destabilization in HID lamps by amplitude modulation of a high frequency signal and pulsing the lamp, which can be used to change the color characteristics of the lamp.
A new class of high intensity discharge lamps has been produced in the past five years with ceramic (polycrystalline alumina) envelopes. Philips Lighting Company of Somerset, N.J., for example, sells such ceramic discharge metal halide (CDM) lamps under the trademark MASTERCOLOR.TM.. The discharge envelope is cylindrical in shape, and the aspect ratio, i.e., the inner length (IL) divided by the inner diameter (ID) is close to one. At least one new type of cylindrical lamp is being developed which has a much larger aspect ratio. Such new lamps have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs. Projecting an image of the arc onto a screen shows that the bottom part of the arc appears pink while the top part looks green. This is caused by the absence of complete mixing of the atomic metal additives in the discharge. In the upper part of the discharge there is too much thallium emission and insufficient sodium emission. The result of vertical segregation is a lamp with higher color temperature compared to horizontal operation and possibly decreased efficacy.